Recent multiple Grammy winner Brandi Carlile and Americana favorite Amanda Shires were known several months ago to be forming a supergroup of sorts, the Highwomen, but who the third member would be remained a mystery. That puzzle was solved Tuesday night when Carlile used her Instagram account to post a group photo letting fans know the lineup was being completed by country star Maren Morris, whose second solo album, “Girl,” comes out this week.

Although no official announcement has been made, the declaration seemed clear from the “Highwomen” caption alongside the photo of Carlile, Shire and Morris (and a Rosie the Riveter-style flexed-bicep emoji).

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Highwomen 💪🏼 @marenmorris @amandapearlshires

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Another social media post appeared to make the unofficial announcement that the trio has already been at work recording. A photo of the three in the studio alongside famed Nashville producer Dave Cobb was posted on Twitter, with the caption “Fun at work,” by Jason Isbell, Shires’ 400 Unit bandmate and husband.

Morris didn’t post about the project herself but replied to Carlile’s, writing, “Thank you for diving into (songwriter Natalie) Hemby’s and my old folky demos for some of this. Love you two.”

The project went public in early January when Shires revealed the group’s name in an interview with Louisville radio station WFPK. At that time she seemed to indicate Margo Price would be part of the project, but that was quickly walked back by Price’s publicist and Shires herself. In a mid-January interview with Variety, Carlile said, “We locked in our final member today,” but held who that was close to the vest.

One thing that’s become clear as Shires and Carlile have talked about the project is that they intend it to be closer to straight-up country, something they previously have only skirted in their more folk-rock-oriented singer-songwriter day jobs.

In her interview with Variety, Carlile spoke of wanting to see the gender gap in country music addressed before her daughters grew up and wanted to become musicians. “I need my kids to not see that the last two months has been the first time in 10 years that there are zero women in the top 20 in country music,” she said. “What if they like country music, and there’s one woman on the radio? What message is that going to send them? So that’s what the Highwomen is about.”

(Read Variety‘s February cover story on Carlile here.)

On Carlile’s Instagram page, as other female singer-songwriters replied with shows of support, she invited them to participate in on the project. “C’mon down!” she told Chely Wright. “Get your soul on this thing,” she wrote back to Courtney Marie Andrews.

And to a presumably male troll who filled the comments thread with remarks like “Pistol Annie copycats” and “So, you think three women is an original idea?,” Carlile responded: “You’re the reason we’re doing this.”

The Highwomen moniker is a play on the Highwaymen, the all-male supergroup formed in the mid-1980s by Johnny Cash, Waylon Jennings, Kris Kristofferson and Willie Nelson. In her initial radio remarks about the fledgling band, Shires said she conceived the idea for the group and added that the same was meant to be taken “as in exalted, not stoned. I mean, I’m sure being stoned is fine depending on where you are and all of that. I’m not advocating anything, or un-advocating anything.” Shires mentioned Jim Lauderdale as a co-writer and Sheryl Crow as a likely guest. “Any women that want to come, and men, it’s all inclusive. It’s not like man haters.”

Morris and Carlile sing a duet on the former’s new album and appeared together at the latter’s Girls Just Wanna festival in January. Shires’ seventh album, “To the Sunset,” came out in August, and was co-produced by Cobb, as was Carlile’s triple Grammy winner, “By the Way, I Forgive You.”